Recently, there is a global trend toward stricter quality regulation values for gas oils so as to improve atmospheric environments. In particular, a sulfur reduction in gas oils is desired because there is a fear that the sulfur components contained in gas oils may adversely influence the durability of after-treatment technologies expected to be effective for exhaust control, such as oxidation catalysts, nitrogen oxide (NOx) reduction catalysts, and continuous-regeneration type catalyzed diesel particulate filters for removing particulate matter from diesel exhausts.
Under these circumstances, develop of a technique of ultra-deep desulfurization for removing most of the sulfur components in a gas oil is being focused. A possible technique generally usable for reducing the sulfur components of a gas oil is to use severer operating conditions for hydrodesulfurization with respect to, e.g., reaction temperature and liquid hourly space velocity.
However, use of an elevated reaction temperature results in impregnation of a carbonaceous matter on the catalyst and hence in a rapid decrease in catalytic activity. On the other hand, use of a lowered liquid hourly space velocity results in a reduced purification efficiency although effective in attaining a higher degree of desulfurization, making it necessary to enlarge the scale of the hydrotreater.
Consequently, the best way of attaining the ultra-deep desulfurization of a gas oil without using severer operating conditions is to develop a catalyst having excellent desulfurization activity.
Many investigations are recently being made on various subjects such as the kinds of active metals, methods of active-metal impregnation, improvements of catalyst supports, regulation of catalyst pore structures, and activation methods, and novel catalysts for deep desulfurization developed have been reported.
For example, JP-A-61-114737 discloses a method which comprises impregnating an alumina or silica support with a solution which contains an organic compound having a nitrogen-containing ligand as a complexing agent and further contains an active metal, followed by drying at 200° C. or lower.
Japanese Patent No. 2,900,771 discloses a method which comprises impregnating a γ-alumina support with an impregnating solution obtained by adding diol or ether to a solution containing a compound of a metal in the Group 8 of the periodic table (hereinafter simply referred to as “Group 8 metal”), a compound of a metal in the Group 6 of the periodic table (hereinafter simply referred to as “Group 6 metal”), and phosphoric acid, followed by drying at 200° C. or lower.
Japanese Patent No. 2,832,033 discloses a method which comprises impregnating a support with a solution comprising a compound of a Group 6 metal, a phosphorus component, a compound of a Group 8 metal and citric acid, as in the process of the present invention, followed by burning without drying. JP-A-4-156948 discloses a method which comprises impregnating a support having provided thereon a compound of a Group 6 metal, a phosphorus component and a compound of a Group 8 metal with a solution containing a specific amount of an organic acid, followed by drying at a temperature of 200° C. or lower.
Furthermore, JP-A-4-244238 discloses a method which comprises depositing a solution comprising a compound of a Group 6 metal, a compound of a Group 8 metal, and a phosphoric acid on an oxide support, drying this support at 200° C. or lower to obtain a catalyst, depositing a solution of an organic acid represented by a specific chemical formula on the catalyst, and then drying the catalyst at 200° C. or lower.
On the other hand, proposals have been made also on a process for catalyst production in which impregnation with an organic acid is conducted twice.
For example, JP-A-6-339635 discloses a method which comprises impregnating an oxide support with a solution comprising a compound of a Group 6 metal, a compound of a Group 8 metal, organic acid and phosphoric acid, followed by drying at 200° C. or lower to obtain a catalyst, and further impregnating the catalyst with a solution of organic acid, followed by drying at 200° C. or lower.
JP-A-6-31176 discloses a technique for catalyst production which comprises impregnating an inorganic oxide support with a compound of a Group 8 metal and heteropolyacid of a Group 6 metal, followed by drying.
Moreover, JP-A-1-228552 discloses a process for catalyst production which comprises impregnating an oxide support with a solution comprising molybdenum, tungsten, a compound of a Group 8 metal, mercaptocarboxylic acid and phosphoric acid.
This process is mainly intended to form a coordination compound of the mercaptocarboxylic acid with molybdenum, tungsten, and the Group 8 metal compound and to highly disperse the coordination compound on the catalyst support.
However, in the catalyst produced by the process described above, the molybdenum and tungsten is highly dispersed on the support and, hence, it is difficult to form laminating layers of molybdenum disulfide such as those in the present invention which will be described later. It is presumed that this related-art process yields no Type II sites of a CoMoS phase or NiMoS phase which are especially effective as active sites for desulfurization (i.e., active sites of cobalt or nickel located at the edges of the second and overlying layers of molybdenum disulfide; Type I sites mean the active sites of cobalt or nickel located at the edges of the first layer of molybdenum disulfide, and have lower activity than the Type II sites).
In addition, there is a possibility that when the mercaptocarboxylic acid, which contains sulfur, is present around the Group 8 metal (cobalt or nickel) or form a coordination compound, then the sulfur contained in the acid might give not desulfurization-active sites (CoMoS phase or NiMoS phase) but Co9S8 or Ni3S2, which are inactive forms of cobalt and nickel.
The processes for catalyst production described above have drawbacks that some of these necessitate complicated steps and that some of the catalysts obtained are unsuitable for use in the ultra-deep desulfurization of a gas oil, some have a low efficiency in the ultra-deep desulfurization range, and others have a short life. Accordingly, at the present, there is a desire for the development of a technology for obtaining a catalyst which has higher desulfurization activity and a longer life than desulfurization catalysts heretofore in use and with which the ultra-deep desulfurization of a gas oil can be realized by a simple method without using severer operating conditions.